Try rendering one the upper left hand corner to
perfect the vignetting adjustment from Adobe
Camera RAW, Adobe Lightroom, DxO Optics, Canon or
Nikon imaging software. You can use the Canon
Peripheral Illumination feature in camera or in
post for a select group of supported lenses. On
Nikon you can use the vignetting correction in the
custom camera settings menu for certian lenses.
You can also make a second pass over the smaller
rock cairn with a different focus and then merge
the two images with different focus. For this type
of subject it is best to make multiple passes. I
have made a mess by trying to focus stack
landscapes in one pass position by position.

This is great pano, but very heavy grid of
vignetting makes it not perfect one. I think you
could consider restitching this panorama after
reducing the vignetting with very simple but quite
good freeware www.photo-freeware.net/vignetting-
reducer.php